The present invention relates to the recovery of heat from a gas stream contaminated with water vapor and an acid-forming moiety, and more particularly to a heat exchanger adapted therefor.
In many applications, it is desirable that waste heat from an exhaust gas be used to preheat boiler make-up water, boiler air, or other process fluid. When the exhaust gas is contaminated with water vapor and an acid/acid-forming moiety, such as flue gas, significant problems can develop during the heat recovery operation. Initially, the waste heat typically is recovered by passing the gas stream through a heat exchanger fitted with a plurality of generally parallel tubes through which heat exchanging fluid, e.g. water or air, flow. The art has recognized that the flue gas can be cooled condensed to a temperature above the acid dew point without endangering the internal components of the heat exchanger. Such an operation, however, recovers only a fraction of the available heat from such flue gas waste stream.
In order to recover substantially all of the economically-recoverable heat value in flue gas, the art has devised condensing economizer heat exchangers. Such heat exchangers can be found in the following U.S. Pat. Nos.: 4,738,310, 4,487,139, 4,526,112, 4,577,380, 4,669,530, 4,776,391, 4,557,202, and 4,705,101. The '310 patent uses a glass tube heat exchanger wherein the apertured tube sheets retaining the glass tubes are lined with corrosion-resistant material. The interior volume of the heat exchanger, then, is sufficiently corrosion-resistant that condensed acid can be tolerated, especially since water spray provision for washing the tubes is a standard feature of such condensing economizers. The latter listed patents employ metal tubes that are coated with polytetrafluorethylene (Telflon brand fluoroplastic) in order to make the tubes corrosion-resistant. These patents also utilize multiple heat exchangers arranged in a variety of configurations.
With respect to the use of glass tubes, it will be appreciated that the thermal conductivity of glass is about 0.63 btu/hr-ft-.degree.F. Polytetrafluoroethylene-coated metal tubes must utilize a thin coating (0.15 in.) as such fluoroplastics tend to be somewhat insulative at thicker coating levels, (0.040 in.) and the thinness of such coatings detracts from longevity of the coated tubes. State of the art in chemical processing equipment dictates that 0.040 in. minimum thickness of fluoroplastic is required for general corrosion protection. Thus, there is a need in the art to provide improved corrosion-resistant, durable tubes in condensing economizer heat exchangers.